Composite conductor bar and method of manufacturing

ABSTRACT

An elongated structural steel cap which is slightly smaller than an extruded aluminum conductor bar is forced onto the bar and held in this position by nails explosively driven through the side legs of the cap end into the side walls of the conductor bar. Slots formed in the aluminum conductor bar permit the bar to be laterally distorted as the cap is forced onto the bar so that wide variations in the cap dimensions can be accommodated.

ll mted States Patent 1 1 Scofield et al. 1 my 22, 1973 541 COMPOSITECONDUCTOR BAR AND 3,366,334 1/1968 Broske ..238/151 METHOD OF URIN3,399,281 8/1968 Corl ..191/29 DM x 2,050,047 8 1936 Febre ..227 9 XInventory Dmlald Scofield; James 2,590,585 3h952 Temple 247/9 both ofSan Carlos, Calif. [73] Assignee: Insul-8-Corp., San Carlos, Calif.FOREIGN PATENTS OR APPLICATIONS Filed: Mar. 1971 762,784 12/1956 GreatBritain ..174/94 [21] Appl. No.: 124,551 Primary ExaminerCharles W.Lanham Assistant Examiner-Joseph A. Walkowski Related Apphcation DamAtt0rneyFowler, Knobbe & Martens [62] Division of Ser. No. 771,931, Oct.30, 1968.

[57] ABSTRACT [52] US. Cl. ..29/624, 29/43332,22398/525l, An elongatedStructural Steel cap which is Slightly 191/22 191/ H H 92 smaller thanan extruded aluminum conductor bar is 2; 22 23 forced onto the bar andheld in this position by nails 1 0 a explosively driven through the sidelegs of the cap end I l l into the side walls of the conductor bar.Slots formed in the aluminum conductor bar permit the bar to belaterally distorted as the cap is forced onto the bar so [56] ReferencesCited that wide variations in the cap dimensions can be ac- UNITEDSTATES PATENTS commodated- 3,341,669 9/1967 Martin et al ..191/29 DM 4Claims, 7 Drawing Figures PATENTEDrmzz m5 SHEET 2 BF 3 INVENTORS. JAMES4 021. BYDO/VALD A. 560F750 COMPOSITE CONDUCTOR BAR AND METHOD OFMANUFACTURING This is a division of application Ser. No. 771,931, filedOct. 30, l968.

BACKGROUND OF THE INVENTION This invention relates to electricaldistribution systems wherein a current collector slides in contact withan electrical conductor, and more particularly relates to an improvedcomposite conductor for such a system.

Mobile, or sliding contact, electrification systems are used extensivelyin industry to provide electrical power to mobile apparatus such asbridge cranes, trolley cars, etc. Generally, such systems include aconductor extending along the path of the mobile apparatus, and acurrent collector mounted to move with the apparatus. The conductor hasan exposed face which is slideably contacted by the collector to supplypower to the machinery.

In addition to low electrical resistance, a satisfactory conductor forsuch sliding contact systems must have a surface which makes a goodelectrical connection with the collector shoe and must be able towithstand the constant wear of the sliding collector shoe. Also,structural strength, low weight and low cost are desirable features.Cost is particularly significant in view of the large quantities ofconductor bars required for many electrification systems.

Aluminum conductor bars have a relatively good combination of highstructural strength, low electrical resistance, low weight and low costin comparison with conductor bars of other metals, and are especiallysuitable for heavy duty installations such as steel mills and shipyardgantries where the conductors often must be large enough to carrycurrents on the order of 500 to 1,000 amperes or more. However, aluminumhas severe disadvantages in a sliding contact system because aluminumoxide which forms on its surface is a very poor electrical conductor.Moreover, aluminum is soft and has a relatively high coefficient offriction, hence affords a poor wearing surface for sliding contact.

To overcome these shortcomings of aluminum, while still having thebenefit of its many advantages, composite conductor bars have been madewherein a steel cap is attached to the aluminum bar so that the steelforms the contacting surface'for the current collector. Steel, beingmuch harder than aluminum can, of course, withstand the constant wear ofsliding contact much better thanaluminum One difficulty which isencountered in providing an aluminum-bar with a steel cap is that meansmust be provided to insure that current can be conducted from thealuminumbar to the sliding current collector with a minimum of increasedresistance by virtue of the steel cap. ln one known approach, holes arepredrilled in the side legs of an extruded steel cap and aluminum plugsare then welded through the holes into the aluminum bar so that thewelded plugs securely attach the cap to the bar. The welded aluminumplugs also make good electrical contact with the steel cap withouthaving any undesirable oxidation of the aluminum. Consequently, most ofthe electrical transfer between the conductor bar and the cap is throughthe aluminum welds. While this approach has met with considerablesuccess, it is still naturally desirable that the cost be reduced inthat the predrilling and the actual welding process both add to thecost. Also the steel cap must be extruded to fit properly on theconductor bar while being subjected to extreme temperature variations,bearing in mind the different coefficients of expansion of the twometals. Hot rolled structural steel is less expensive. Another problemis that since most of the current flow is through the spot welds, thereis some tendency for the welds to weaken after extended usage such thatthe cap may not remain securely attached to the conductor bar.

It has also been known that specially formed steel elements be attachedto an aluminum conductor by means of bolts or welds through prepunchedholes which extend completely through the conductor bar and the steelsurface member. Such an arrangement is illustrated in U. S. Pat. No.3,222,464. Again, while this approach has met with some success,elimination of special forming steps can reduce the cost.

In addition to being inexpensive, the means for attaching the cap to thebar should be simple so that it can be performed by relatively unskilledworkers, and it naturally must be extremely reliable to meet the ruggedoperating conditions which such composite conductor bars may besubjected.

SUMMARY OF THE INVENTION Briefly stated, the composite conductor bar ofthe invention includes an elongated bar of good electrically conductivematerial such as aluminum having a relatively large cross sectionalarea, the bar having a front wall, and a pair of opposite side walls. Agenerally U- shaped steel cap is forcefully engaged onto the conductorbar with its inner surface engaging the front and side walls of the bar.The cap is then securely attached to the bar by means of explosivelydriven nails which penetrate the side legs of the steel cap and embed inthe aluminum side walls of the conductor bar. Nails are installed atspaced locations on both sides of the cap. Such nailing can beaccomplished in an inexpensive manner without requiring any predrillingor other operation.

In the preferred form of the invention, the aluminum bar is formed withtapered side walls and with one or more pair of slots adjacent the sidewalls of the bar, the slots extending generally perpendicular to thefront face of the conductor bar. With this construction, a steel cap isformed with a slightly smaller taper on the inner surface of the legsand with its inner width slightly smaller than the correspondingdimension of the conductor bar so that the cap must be forced onto thebar. The slots within the bar permit the bar to be laterally distortedso long as there is space into which the aluminum can move when thepressure is applied. The result is that the cap is then in goodmechanical and electrical contact with the bar. While the cap is held inforceful engagement with the bar, a series of nails are driven into thecomposite conductor. The excellent contact is thus maintained in view ofthe tendency of the aluminum bar to attempt to spring back somewhat toits original shape. A major advantage of this construction is that lowcost structural steel channels can be satisfactorily employed.

To further improve the electrical contact between the steel cap and thealuminum bar, a layer of conductive grease is applied to the bar beforethe cap is installed. The bar with the coating of grease is then brushedwith steel brushes so as to scour the external surface and remove muchof any aluminum oxide which may be present on the surface of the bar andto embed conductive grease into the unoxidized aluminum surfaces exposedby the brushing operation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of thecomposite conductor of the invention with one end of the conductor beingshown in cross section to illustrate its construction;

FIG. 2 is an enlarged cross sectional view of the conductor bar beforethe steel cap is applied to the bar;

FIG. 3 is a perspective view of the bar of FIG. 2 illustrating theconductive grease applying step of the method of the invention;

FIG. 4 is a view similar to FIG. 3 illustrating the brushing operationfor removing aluminum oxides;

FIG. 5 is an elevational view of the end of the composite conductor barand a view of the pressing equipment and nailing apparatus illustratingthe manner in which the nailing operation is performed;

FIG. 6 is an enlarged cross sectional view of another embodiment of theconductor bar of the invention before a cap is applied; and

FIG. 7 shows the cap of FIG. 6 after a bar is forced thereon.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Referringfirst to FIG. 1, the electrical conductor 10 of the invention may beseen to include an elongated aluminum bar 12 which is extruded with across sectional shape having a basic outline somewhat like that of anl-beam. The bar 12 includes a base 14 a central column 16 and anenlarged head 18 which represents the top portion of the so-calledI-beam cross section.

Enclosing the top or front and side walls of the head 18 is an elongatedstructural steel channel or cap 20 which forms a wearing surface for thecomposite conductor 10. In accordance with the invention the generallyU-shaped cap 20 is held in place by a plurality of longitudinally-spacednails 24 extending through the side legs 21 and 22 of the cap and intothe head 18 of the conductor bar 12.

The primary use for the conductor bar 10 is for conducting electricityto mobile equipment such as overhead cranes, hoists or similar equipment(not shown). The electricity is taken from the conductor by means of alarge conductive collector shoe 26 mechanically connected to an arm 27which is attached to the equipment using the electricity so that theshoe slides along the front or exterior surface of the center web 23 ofthe cap 20. The shoe is connected electrically by cables 29 to theequipment. The steel cap is employed to provide a wearing surface harderthan aluminum which is a better conductor. Since it is necessary thatthe cu-rent pass from the aluminum bar into the steel cap, it isessential that good electrical contact be made between the bar and thecap.

The aluminum conductor bar 12 is initially extruded with its headportion 18 having the relatively large cross section illustrated in FIG.2. This includes a generally flat top or front wall 30, a pair ofopposite side walls 31 and 32, and a rear wall 33 formed in two sections33a and 33b separated by the vertical column 16. The side walls 31 and32 are formed to taper outwardly from the front wall to the rear wallsections 33a and 33b. Although various tapers can be employed, as willbe discussed below, a preferred amount is approximately 12 with respectto the vertical as viewed in FIG. 2.

A pair of elongated slots 38 and 39 are formed in the bar with the slot38 being spaced inwardly from the side wall 31 and the slot 39 beingspaced inwardly from the side wall 32. The slots 38 and 39 are orientedgenerally vertically as viewed in FIG. 2 and extend substantiallyperpendicularly with respect to the front wall 30. As can be seen, theopen ends 38a and 39a of the two slots open to the front wall of the barwhile the closed ends of the slots extend downwardly to a depth slightlymore than one-half the distance to the rear wall 33. The side walls ofthe slots 38 and 39 taper outwardly slightly from the smaller closedends 38b and 39b at an angle of approximately 5 with respect to thevertical as they extend to the front wall 30.

A second pair of slots 42 and 43 are also formed in the head 18 of theconductor bar 16 extending approximately parallel to the slots 38 and 39and positioned between the side walls 31 and 32 and the slots 38 and 39.The lower ends of the slots 42 and 43 open respectively to the rearwalls 33a and 33b while the closed ends 42b and 43b extend upwardly tobe closer to the front wall 30 than the closed ends 38b and 39b of theslots 38 and 39. Thus in effect the closed ends of the slots overlap ina vertical direction without intersecting. Consequently, some portion ofa slot extends from the front wall to the rear wall for the entirelength of the side walls 31 and 32.

The side portions of the conductor bar head 18 are essentially formedinto wings 46 and 47 composed of sections 46a and 46b and 48a and 48b.Section 46a is attached in cantilever fashion at its upper or front endto the upper or front end of the section 46b. The section 46b in turn isconnected in cantilever fashion to the central portion of the head 18.Similarly, the outer section 48a on the conductor bar wing 48 isconnected in cantilever fashion at its upper end to the inner section orcolumn 48b, which in turn is connected in cantilever fashion at itslower end to the central portion of the conductor head 18. As a resultof the slots and the cantilever connections formed thereby, the wings 46and 48 can be distorted or telescoped inwardly somewhat inaccordion'fashion if sufficient force is applied.

Since it is difficult to conduct electricity between an aluminum andsteel interface, there are some intermediate steps to be taken.Referring to FIG. 3 there is shown a dispensing nozzle 54 through whicha conductive grease 56 is applied onto the top or front surface of theconductor bar. There are a variety of such grease-like conductivematerials presently available. In addition to being highly conductive,they are quite viscous and have the capability to withstand relativelyhigh temperatures. One example of a suitable grease presently availableis sold by Alcoa under the identification EJC-2.

As the next step there is shown in FIG. 4 a horizontally mountedcircular brush 58 having bristles made of stainless steel engaging thefront surface 30 of the conductor bar. A similar pair of circularbrushes 60 are mounted to engage the front corners 34 and 35 of theconductor bar and another pair of brushes 62 are generally verticallyoriented to engage the side walls of the conductor bar. The brushesslowly rotate at a speed of approximately about 200 revolutions perminute while the conductor bar is drawn between the rollers.

The brushes scour the surfaces of the front and side walls of theconductor bar removing much of any aluminum oxide formed thereon.Secondly, the brushes distribute the conductive grease so that the frontand side walls are substantially coated with the grease. Since thealuminum oxide is removed by the brushing operation, the grease is thusplaced directly in contact with the unoxidized aluminum. Since aluminumoxide is a good insulator, this operation therefore eliminates thislayer of insulation, prevents additional oxidation of the aluminum, andprovides a conductive medium to compensate for any imperfections in themechanical connection between the mating surfaces.

As the next step of the operation, the steel cap is to be forced ontothe conductor bar 12 to the position shown in FIGS. 1 and S. The steelcap should preferably have been previously scoured or sandblasted toremove any scale or foreign matter which may have accumulated on thecap. As previously explained, the cap is made of a low cost structuralsteel channel which has been hot rolled formed, this being the leastexpensive type steel channel available. Channels formed by hot rollingfrequently have considerable scale collected on their exterior surfacesparticularly if they have been in storage for a considerable period oftime. In addition to a mechanical scouring step, a chemical bath may beused if desired to further clean the metal.

The side legs 20 and 21 of the structural steel cap have exteriorsurfaces which are approximately perpendicular to the exterior surfaceof the center web 23. The inner generally U-shaped surface of the capincludes a flat inner surface on the center section 23 which isapproximately parallel to the outer surface of the center section. Theinner surfaces of the side legs 21 and 22 however taper outwardly fromthe center web 23 at an angle of approximately 8, as illustrated in FIG.5. This taper occurs as the result of the rolling equipment employed informing the channel.

The front end of the conductor bar 12 is extruded to a dimension smallerthan the entry to the open end of the steel cap so that the cap will fitpartially onto the conductor bar. However, since the side walls 31 and32 of the conductor bar taper outwardly at an angle of approximately12", further downward movement of the cap results in interferencebetween the cap legs, having an 8 taper, and the side walls of theconductor bar before the inner surface of the cap center web 23 isbottomed or engaged on the front wall 30 of the conductor bar.Consequently, it is necessary to force the cap onto the bar by means ofexternal pressure.

As shown in FIG. 5, this is accomplished by placing the conductor bar ona supporting surface 64 and applying pressure to the cap 20 by suitablemeans such as a hydraulic piston 66 and pressure plate 68. Equipmenthaving a pressure capability of 30,000 pounds per lineal foot ofconductor bar has been found to be more than adequate to press the caponto the bar. It is understood of course that in view of the anglesinvolved with the cap and the side walls of the conductor bar,considerably less force than 30,000 pounds is actually applied to theside walls of the conductor bar. In operation, the pressure can beapplied to the cap and the bar in various ways. If desired an entiresection of cap can be pressed onto a rail at a given time if sufficientpressure equipment is employed, or, more practically, a pair of pistonsand pressure plates spaced a convenient distance apart along the railcan be employed to press sequentially sections of the cap onto theconductor bar.

Hot rolled structural steel channels such as that employed for the steelcap 20 have considerable tolerances in their dimensions between theinterior surfaces of the side legs 21 and 22. This is not only true fromone batch of steel to another and from one manufacturer to another butalso considerable variation can occasionally be found on a given sectionof steel channel. In accordance with the invention, the conductor barhas been designed to receive a channel or cap having the smallestexpected internal dimension for a given nominal size of channel and toalso receive the channel having the largest expected dimension whilestill obtaining good electrical and mechanical contact between the capand the bar. In practice, it has been found that the lateral variationof the interior dimension of the cap or channel 20 may be more thanonefourth of an inch. Consequently, the combined width of the slots 38,39, 42 and 43 is equal to or greater than this amount. In a preferredexample the slots were made with a closed end radius of approximatelyoneeighth, widening to one-fourth of an inch at the open end.

In accordance with the invention, the aluminum conductor bar has beendesigned to accommodate such variations. As the steel cap is forceddownwardly by the hydraulic piston 66 and its pressure plate 68, theside legs 21 and 22 of the steel cap engage the side walls 31 and 32 ofthe aluminum bar and force or distort the wings 46 and 48 laterallyinwardly to the position shown in FIG. 5 wherein the entire innersurface of the steel cap is in good electrical and mechanical contactwith the front wall 30 and side walls 31 and 32 of the conductor bar. Ascan be seen by comparing FIG. 2 and FIG. 5, the slots 38, 39, 42 and 43permit this lateral distortion. A completely solid conductor bar can ofcourse not be compressed or distorted in such manner. The importantpoint is that space must be provided for the aluminum to move whereverthere will be interference between the cap and the conductor bar.

As a result of the interference between the cap and the conductor, goodelectrical and mechanical contact is made between large areas of theconductor bar and the steel cap. One side factor contributing to thegood electrical contact is that the legs of the cap 20 tend to wipe orscrape the side walls of the conductor bar as the cap is moved onto thebar; hence advantageously any oxidized aluminum which may have remainedafter the brushing operation is likely to be removed in the area scrapedby the cap.

As the next step of the invention, it is necessary to secure the cap inthe position wherein it is in forceful engagement with the conductorbar. The conductor bar has considerable springiness or memory such thatif the pressure on the steel cap is removed before securing the steelcap to the conductor bar, the steel cap would pop upwardly part way offof the conductor bar. Accordingly, it is necessary that pressure bemaintained while the cap is being attached to the conductor bar. Thenail tips 24 are driven through the side legs of the steel cap andembeded in the conductor bar with the nail head engaging the outersurface of the cap leg. Such an operation can be performed by anexplosively powered tool shown in FIG. 5. The tool includes a piston 72which is driven forwardly by triggering an explosive charge to drive thenail 24 into the cap and the conductor bar.

The pressing and nailing step is repeated for the length of the bar withthe nails preferably spaced between 6 and 12 inches apart on both sidesof the steel cap, and they are staggered between the sides. The nailsshould be located sufficiently far from the free ends of the cap legs toobtain sufficient strength and to prevent distortion of the cap fromappearing along the lower edge of the cap legs. Preferably, the nailshould be about one-half inch from the free ends of the cap leg with thecap illustrated.

Note from FIG. that the nails 24 extend completely through the side legsand the outer sections 46a and 48a of the conductor bar. Depending uponthe dimensions involved the points of the nails may partially penetratethe next sections of the bar.

It has been found that the nails 24 hold the cap to the aluminumconductor bar in a very positive fashion. The nails themselves providemost of the holding force in friction with the aluminum and steel, butin addition it should be noted from FIG. 5 that the inner portion 74 ofthe cap leg surrounding the nail is extruded or cold formed into amating recess in the side wall of the conductor bar. That is, theconductor bar is distorted by the portion 74 of the cap extruded intoits surface. This extruded portion 74 also adds to the positive natureof the attachment obtained.

In addition, it is believed that the frictional heat developed as thenail is driven through the two metals causes expansions in the latticestructure of the adjacent metal molecules, with the result thatinterdiffusion between the molecules develops. This, in turn, adds tothe strength of the connection. This positive connection plus the springback characteristics of the aluminum enable good electrical contactbetween the cap and the conductor to be maintained throughout thevarious temperature extremes to which a conductor rail of this type maybe subjected bearing in mind the different coefficients of expansion foraluminum and steel. It should be appreciated also that good electricalcontact is maintained substantially throughout the entire inner surfaceof the steel cap and the mating surfaces of the conductor bar in view ofthe pressure applied to the cap and the bar during the nailingoperation. Also the presence of the conductive grease compensates forany mechanical imperfections in the mating surfaces.

In a preferred example of the invention, the aluminum alloy used for theconductor bar is that identified as ASTM No. 6063-T6, which hasexcellent conductivity characteristics. The steel cap employed was 1018hot rolled structural steel.

The nails employed must of course be harder than the steel cap in orderto penetrate the cap. Also, they should have sufficient size towithstand the forces involved. With a conductor bar having a headdimension of approximately lh inches by 2% inches and the cap having legthickness of about one-fourth inch at the point of penetration, a nailhaving an overall length of about three-fourths inches and a diameter ofabout five thirty-seconds inches has been found satisfactory.

While a variety of explosively driven nailing tools are currentlyavailable on the market, a model found to be particularly satisfactoryis sold under the trademark Hilti made by Fastening Systems of Stamford,Connecticut, the model number being DX 100-L. Generally similarequipment is sold under the trademark Ramset by Olin Mathieson ChemicalCorporation.

The composite conductors are typically shipped in sections to the sitewhere they are to be used and then assembled into the necessary lengths.Splices or joints between adjacent sections of the conductor bar areconveniently formed by attaching splice sections to adjacent centralcolumns 16 of the adjacent conductor rails. The rear wall sections 33aand 33b together with the upper surfaces of the base 14 are convenientlytapered to facilitate insertion of the splice sections.

EMBODIMENT OF FIG. 6

In FIG. 6, the head 80 of an aluminum conductor bar is extruded with asingle pair of elongated slots 81 and 82 which open to the rear wall 83of the bar and extend generally perpendicular to the front wall 84. Theclosed ends 81a and 82a of the bar terminate adjacent to but spaced fromthe front wall. By way of example, in one arrangement this distance isabout three-eighths of an inch, with the slot being about an inch inlength and tapering outwardly slightly from the closed ends to rearwall. The slots define a pair of side wings 85 and 86 attached incantilever fashion at their upper ends to the central portion of thehead 80.

The corners 87 and 88 between the front wall 84 and the sidewalls 89 and90 gently curve through an are having a radius of approximatelythree-fourths of an inch. The sidewalls 89 and 90 taper outwardly towardthe rear wall 83 at an angle of approximately 12 with respect to thevertical, which is perpendicular to the front wall 84.

Referring to FIG. 7, the cap 92 is identical to the cap discussed inFIGS. l5 having a generally U-shaped interior surface including sidelegs 93 and 94 having interior surfaces which taper outwardly towardsthe open end of the cap at an angle of approximately 8 with respect tothe vertical, which is perpendicular to the exterior surface of the web95 of the cap. The exterior surface of the cap and the interior surfaceof the cap web are generally flat. The interior comers 96 and 97 arecurved more sharply than the curved corners 87 and 88 of the bar, beingabout one-fourth to three-eighths inch in the example given. Thedimensions of the conductor bar head are such that the open end of cap92 will fit partially onto the head 80 so that the cap legs will engagethe corners 87 and 88 or the sidewalls 89 and 90 of the conductor bar ata point below the closed slot ends 81a and 82a. Further movement isresisted because of the interference caused by the differing tapers onthe conductor bar and the cap. It should be understood that the greasingand scouring steps discussed above are performed on the bar before thecap is placed on the bar.

By forcing the cap onto the conductor bar through pressure applied tothe cap, the side legs 93 and 94 of the cap force the cantilever wingsand 86 on the conductor bar inwardly about their upper ends to theposition shown in FIG. 7 wherein the side legs 93 and 94 of the capforcefully engage the sidewalls 89 and of the conductor bar and theinner wall surface of the cap web engages the flat upper or frontsurface 84 of the conductor bar. This inward distortion of the wings 85and 86 is permitted by virtue of slots 81 and 82 formed in the conductorbar. Since the corners 87 and 88 of the conductor bar are more gentlycurved than the inner corners 96 and 97 of the cap 92, the cap as shownin FIG. 7 does not completely forcefully engage the corners 87 and 88 ofthe conductor bar. Note also that the slot ends 81a and 82a spacedapproximately three-eighths of an inch from the wall 84 overlap orextend into the area defined by the three-fourths inch radius formingthe curve for the corners 87 and 88. Consequently, the corners and theslots together permit the wings to be distorted inwardly sufficiently sothat caps varying considerably in width can be forced onto the conductorbar in good contact with the bar. The important point is that there bespace for the aluminum to be distorted into when forced by the cap. Ifthe cap 92 were to meet the resistance of solid aluminum across theconductor bar, the cap could not be forced onto the bar with the forcescontemplated.

A significant advantage of the arrangement of FIGS. 6 and 7 is thatemploying a single pair of slots in the conductor bar presents a simpleextrusion process for the bar. Although the amount of good surfacecontact between the conductor bar and the cap is on the average perhapsslightly less than in the previous embodiment, it will be appreciatedthat very substantial areas of the bar are still in good contact withthe cap.

The cap is held in forceful engagement with the conductor bar byexplosively firing a plurality of nails 98 through the legs of the capand into the wings 85 and 86 of the conductor bar, as in the previousembodiment. Note, however, that the wings 85 and 86 are sufficientlythick that the nail 98 terminates within the wing.

What is claimed is:

1. The method of making a composite electrical conductor formed of anelongated bar made of good electrically conductive material and aferrous metal cap providing a hardened surface for contact with asliding current collector shoe comprising the steps of:

extruding an elongated aluminum conductor bar with a pair of slots eachspaced from a respective one of the opposite side walls of the barextending somewhat parallel to the side walls;

forming said cap with a generally U-shape with the cap side legs beingspaced to create an interference fit with the bar;

forcing the cap onto the bar with the inner surfaces of the cap engagingthe front and opposite sides of the conductor bar, the side walls of thebar being laterally distorted as allowed by the presence of the slots;and

nailing the side legs of the cap to the side walls of the conductor barwhile the cap is held under pressure in forceful engagement with theconductor bar, the nailing being accomplished by firing the nails bymeans of explosives through the cap legs and into the conductor bar sothat the cap remains in good electrical and mechanical contact with theconductor bar when the force applied to the cap and the bar is released.

2. The method of claim 1 including the steps of:

applying a coating of conductive grease to the conductor bar beforeinstalling the cap onto the bar; and

brushing the conductor bar to remove oxides and to apply the conductivegrease to unoxidized surfaces of the conductor bar.

3. The method of claim 1 wherein the step of forcing the cap intoengagement with the conductor bar includes the steps of:

placing the conductor bar against a solid support surface;

placing the cap onto the conductor bar; and

applying pressure against the front of the cap towards the bar to forcethe cap onto the bar.

4. The method of making a composite electrical conductor formed of anelongated bar made of good electrically conductive material and a capmade of electrically conductive material which is harder than the barproviding a hardened surface for contact with a current collector shoecomprising the steps of:

extruding the bar with elongated slot means for per mitting distortionof the bar;

forming the cap with said surface for contacting the collector shoe andwith leg means for creating an interference fit with the bar;

forcing the cap into engagement with the bar, said slot means permittingdistortion of the bar to accommodate the interference caused by the legmeans; and

nailing the cap and the bar together while the cap is held underpressure in forceful engagement with the bar so that the cap remains ingood mechanical and electrical contact with the bar when the forceapplied to the cap and the bar is released, the nails being positionedsuch that they extend generally transversely to the leg means and theslot means.

1. The method of making a composite electrical conductor formed of anelongated bar made of good electrically conductive material and aferrous metal cap providing a hardened surface for contact with asliding current collector shoe comprising the steps of: extruding anelongated aluminum conductor bar with a pair of slots each spaced from arespective one of the opposite side walls of the bar extending somewhatparallel to the side walls; forming said cap with a generally U-shapewith the cap side legs being spaced to create an interference fit withthe bar; forcing the cap onto the bar with the inner surfaces of the capengaging the front and opposite sides of the conductor bar, the sidewalls of the bar being laterally distorted as allowed by the presence ofthe slots; and nailing the side legs of the cap to the side walls of theconductor bar while the cap is held under pressure in forcefulengagement with the conductor bar, the nailing being accomplished byfiring the nails by means of explosives through the cap legs and intothe conductor bar so that the cap remains in good electrical andmechanical contact with the conductor bar when the force applied to thecap and the bar is released.
 2. The method of claim 1 including thesteps of: applying a coating of conductive grease to the conductor barbefore installing the cap onto the bar; and brushing the conductor barto remove oxides and to apply the conductive grease to unoxidizedsurfaces of the conductor bar.
 3. The method of claim 1 wherein the stepof forcing the cap into engagement with the conductor bar includes thesteps of: placing the conductor bar against a solid support surface;placing the cap onto the conductor bar; and applying pressure againstthe front of the cap towards the bar to force the cap onto the bar. 4.The method of making a composite electrical conductor formed of anelongated bar made of good electrically conductive material and a capmade of electrically conductive material which is harder than the barproviding a hardened surface for contact with a current collector shoecomprising the steps of: extruding the bar with elongated slot means forpermitting distortion of the bar; forming the cap with said surface forcontacting the collector shoe and with leg means for creating aninterference fit with the bar; forcing the cap into engagement with thebar, said slot means permitting distortion of the bar to accommodate theinterference caused by the leg means; and nailing the cap and the bartogether while the cap is held under pressure in forceful engagementwith the bar so that the cap remains in good mechanical and electricalcontact with the bar when the force applied to the cap and the bar isreleased, the nails being positioned such that they extend generallytransversely to the leg means and the slot means.